I kind of thought like that at first, too. I thought the name Nero was very conveniently picked. But that could just be it; Capcom being too lazy to think of anything else, or not having a cool alternative.
Nero being a human while having white hair and extraordinary power even without the Devil Bringer arm, well, that doesn't sound right to me. Not to mention that Sanctus calls Nero a ''descendant of Sparda's blood'', says he has ''indeed inherited Sparda's power'', etc. As far as I know, you can't inherit anything by gaining a demonic arm from Vergil's soul. Then Berial tells Nero he's ''just like he was''', referring to Sparda. No, I believe he's supposed to be very similar to Sparda. I think he's part of the bloodline, though maybe not a direct descendant of Sparda, as that would mean Vergil or Dante fathered Nero when they were eleven. Sparda can't be Nero's father since he died before Dante and Vergil had their eighth birthday. If Sparda died just before they became eight, that would mean Nero is around 22 instead of 19 as Capcom said. But I'm pretty sure I heard Dante say he never knew his father. That probably means Sparda died quite a while before.
Nero's arm isn't blue. If it were, I'd say Vergil had something to do with it. Vergil's soul does seem to echo from the Yamato a bit, though. Maybe that's where Nero Angelo went. Not surprising, since swords often have souls in Japanese folklore. I even doubt it's Vergil's soul. Wasn't the Yamato forged by Sparda? If so, the power-hungry part of Sparda's soul is probably inside Yamato. The Rebellion might house the more emotional part of Sparda's soul. I wouldn't be surprised if Sparda set it up that way, so that Dante and Vergil would split everything (the amulet too), so that the Hell Gate wouldn't be opened easily. Guess that backfired.
It's all very weird, and I doubt Capcom even knows where Nero came from, otherwise they would just tell us instead of saying ''wouldn't you like to know'' for years on end. It seems he may have been intended to be a half-demon like Dante and Vergil (my avatar). And the first trailers and artwork of DMC4 didn't show Nero: only Dante. Which would explain why half the game consists of backtracking. DMC4 is a mess, just like DMC3 kind of messed up the DMC1 story of Dante not having seen Vergil since they were both eight.
I hope Capcom gets its act together, or just hires new staff, and new everything.